1. Field of the Invention
The invention concerns a progressive lens improved so as to take account, among other things, of new working conditions that may be encountered by increasingly large numbers of persons wearing prescription eyeglasses.
2. Description of the Prior Art
So-called "progressive" lenses are well-known in the field of ophthalmic lenses. A lens of this kind comprises two substantially constant power areas for far vision and near vision, respectively, and between these two areas a third area of progressively varying focal power. In practise an area of a progressive lens is considered to be of constant power if the power at each point does not differ by more than a specific amount, for example 0.12 diopters, from its nominal value. In practise the substantially constant power area corresponding to far vision is disposed in the upper part of the eyeglass lens and the constant power area corresponding to near vision is disposed in the lower part of the same lens, these two areas being joined by the progressively varying power area without any surface discontinuity. Thus between the two substantially constant power areas there is a region whose power varies continuously between the two power values of the aforementioned two areas. This region corresponds to viewing distances intermediate the viewing of far objects and the viewing of near objects.
It has been known for some time that such lenses are ill-suited to certain tasks. A refined investigation of this phenomenon has indicated that nowadays more and more people need to accommodate to prolonged observation at medium distances, corresponding to neither of the usual stable vision areas. This type of problem, of relatively recent origin, is encountered in particular when an eyeglass wearer has to work regularly and for relatively long periods at a computer terminal or word processing machine visual display screen. In this case the eyeglass wearer is obliged to view the screen on the primary position of his sight axis, that is to say in a direction that is inclined slightly downwards relative to the horizontal. Also, the screen is situated at an intermediate distance, in particular as it is separated from the eyeglass wearer by a keyboard, which the eyeglass wearer views along the normal axis for viewing near objects and at a distance substantially equal to the near vision distance.
Known progressive lenses do not suit such requirements. Doing so is the object of this invention.